A photonic crystal is a crystal in which two or more materials having different indexes of refraction are periodically arranged in order of optical wavelength (usually 0.3 to 0.7 μm), in one dimension, or two dimensions, or three dimensions. The photonic crystal has the effect of strong confinement of light ascribable to a photonic band gap, and by taking advantage of this confinement effect of light, applications in various optical elements or miniaturized optical circuits are expected. It is also known that by employing a line defect in the photonic crystal, it is possible to form an optical waveguide in the photonic crystal.
However, when light directly enters the photonic crystal from a surrounding air, the amount of reflection by the crystal surface is great, and there are many cases in which light cannot travels into the crystal efficiently. Moreover, even when light travels into the optical waveguide formed in the photonic crystal, no highly efficient method for controlling light so as to enter the crystal has been established. For example, as described in non-patent reference document 1 (Electronics Letters from M. Tokushima et al., 2001, Vol. 37, No. 24), the transmission loss was an extremely large 40 to 50 dB.
[Non-Patent Document 1]
                M. Tokushima et al., Electronics Letters, pgs. 1454-1455, 2001, Vol. 37, No. 24        
[Non-Patent Document 2]                J. Ushida et al., Applied Physics Letters, pgs. 7-9, 82, 7 (2003)        